1. Technical Field
This invention relates to turbine engine rotor assemblies in general, and to rotor assembly shrouds in particular.
2. Background Information
A typical gas turbine engine includes a fan, compressor, combustor, and turbine disposed along a common longitudinal axis. The fan and compressor sections work the air drawn into the engine, increasing the pressure and temperature of the air. Fuel is added to the worked air and burned within the combustor. The combustion products and any unburned air, hereinafter referred to as core gas flow, subsequently powers the turbine and exits the engine producing thrust. In most cases, the turbine comprises several stages each having a rotor assembly and at least one stationary vane assembly. The core gas flow causes the rotor assemblies to rotate, thereby enabling the rotor assemblies to do work elsewhere in the engine. The stationary vane assemblies located forward and/or aft of the rotor assemblies guide the core gas flow entering and/or exiting the rotor assemblies.
A shroud is disposed radially outside of the rotor assembly for sealing between the turbine case and the rotor assembly. The shroud includes a blade outer air seal generally formed from a plurality of segments disposed side by side around the circumference of the rotor assembly. The blade outer air seal segments are suspended in close proximity to the tips of the rotor blades.
The extremely high temperature of the core gas flow passing through the turbine necessitates cooling within many of the turbine components. This is particularly true for blade outer air seals. The shroud components are cooled by air bled off the compressor at a temperature lower and a pressure greater than that of the core gas flow. There is a trade-off using compressor worked air for cooling purposes, however. On the one hand, the bled air cools where access is provided and the higher pressure of the bled air prevents detrimental in-flow of hot core gas. On the other hand, air bled off of the compressor does not do as much work as it might otherwise and consequently decreases the efficiency of the engine. This is particularly true when excessive bled air is used for cooling purposes because of undesirable leaks in the cooling path.
Blade outer air seal segments may be biased within the shroud to ensure proper sealing between the blade outer air seal and whatever hardware is adjacent the seal, and to prevent detrimental vibration. Vibration can cause blade outer air seal segments to wear prematurely. Some prior art shrouds use a ring to aggregately bias the blade outer air seal segments around the circumference of the shroud. A difficulty with this approach is that segments will vary in size within their tolerance range. If, in the assembly of the shroud, several "full" segments are placed adjacent a "thin" segment, the biasing force of the ring may not be applied to the thin segment as completely as it is applied to the full segments. As a result, a space between the thin segment and the ring may be created that provides an undesirable leak path for bled air. In addition, the thin segment may be more readily excited, and therefore prone to vibration.
The leakage and vibration problems caused by the tolerance range of the segment widths can be resolved by machining all of the segments together as an assembly to produce a single machined surface. Machining the blade outer air seal as an assembly is, however, a difficult and expensive task. In addition, if one or more of the "machined" blade outer air seal segments later needed to be replaced, that replacement would have to be custom machined as well.
Hence, what is needed is a rotor assembly shroud that uses a minimum of bled air, one that is durable, one that is easily maintained, and one that utilizes readily replaceable pans.